I booked my tickets online through Buquebus. You can also go to the Buquebus terminal on Avenida Córdoba in Puerto Madero and buy them in person. I would recommend buying tickets early. At roughly US$120 round trip, it was an expensive day trip. The price can be much lower using the slow ferry but that takes about three hours one way.
It’s good to show up about an hour before departure. I went on a very busy morning 30 minutes early and the queues were ridiculous. I had to wait 15 minutes to check in and get my boarding pass, then another 15 minutes to go through passport control. Luckily, you go through passport control for both Argentina and Uruguay when you board at either location. That saves a lot of time. Once on the boat, seating is a free-for-all. I was with a group of four and because we were some of the last few people on the boat, we had trouble finding seats together.
Upon arrival in Uruguay, you have to wait in another queue to scan any bags you brought. Once out of the ferry terminal, go to the street and turn left. This street will take you to the historic center of Colonia del Sacramento in about 15 minutes. There are a few signs pointing you in the right direction just in case.
One of the great things about visiting Colonia on a day trip from Buenos Aires is that you don’t have to change money. Everywhere we ate or shopped at in Colonia accepted currency from Uruguay, Argentina, and US Dollars.
A note on planning for time: A half day is more than enough in Colonia. We took the 8:30am ferry and arrived at 10am. Our ferry back to Buenos Aires wasn’t until 8pm. We were ready to go by about 4pm. Check the ferry schedule and allow yourself an appropriate amount of time.